The use of calcium hydroxide pretreatment to overcome agglomeration of technical lignin during fast pyrolysis

Abstract

Technical lignin extracted from lignocellulosic biomass melts and agglomerates when heated, making its thermal processing problematic and impeding this pathway to low-cost renewable aromatics. We have developed a simple pretreatment of technical lignin with Ca(OH)₂ that made possible the continuous pyrolysis of lignin in a fluidized bed reactor to produce a phenolic-rich bio-oil. The yield of bio-oil was approximately 38 wt% for pyrolysis in the temperature range of 450 to 600 °C. Average molecular weight of bio-oil was less than 288 Da indicating that the lignin was extensively depolymerized to phenolic monomers and dimers. Char from pyrolysis of pretreated lignin was fine powder in contrast to the large agglomerates of char produced from untreated lignin. The surface area of char from pyrolysis of pretreated lignin was relatively less than for char obtained from pyrolysis of whole biomass. The melting and agglomeration behavior of lignin appears to be due to the presence of phenolic hydroxyl, carboxylic acid and aldehyde groups in lignin. Pretreatment with Ca(OH)₂ reduced these functional groups by forming hydroxylcalcium phenoxides, phenolic alcohols, and phenolic carboxylate salts that inhibited agglomeration.